IT professional William Muk had just passed the 11km-mark of Sunday's 2XU Compression Run marathon when a runner in front of him suddenly dropped to the ground.
"I thought he had tripped and stopped to help him up," Mr Muk, 51, told The Straits Times. "But then I realised that his eyes were closed and he was not responding."
The incident occurred at about 5am, on the park connector along the Geylang River. An ambulance arrived about half an hour later, and the runner, 45-year-old Martynn Koh Tong Yong, was taken from Stadium Link to Tan Tock Seng Hospital.
Mr Muk noted that Mr Koh was still breathing and had a heartbeat after he collapsed. He was later pronounced dead at the hospital.
Police were alerted to the case at 7.21am and are investigating it as an unnatural death.
According to his Facebook page, Mr Koh was a regional sales vice-president with American-based company Rite-Hite, which specialises in dock equipment and safety systems. He had taken part in the Standard Chartered Marathon Singapore last December and the Sundown Marathon in June.
Relatives and friends told Chinese paper Shin Min Daily News that Mr Koh was an avid marathon participant who loved to help others.
His younger cousin, who wanted to be known only as Ms Koh, told The Straits Times at his wake in Tampines that he leaves behind a wife, a daughter in secondary school and a son in primary school. "It really came as a shock to them... it's really hard for them to cope," said Ms Koh.
Mr Muk added that it was difficult to contact the organisers for assistance as there was no emergency contact number provided on the race bibs, website and guide. Organiser Pink Apple did not respond to queries by press time.
There have been other marathon-related deaths in recent years. In November last year, a 25-year-old female engineer collapsed during the 10km Nike We Run and died in hospital.